1. Field of the Invention
The present invention relates to an ion bombardment or implantation method and apparatus and, more particularly, to a high current ion bombardment or implantation method and apparatus utilizing a rotating disc arranged to operate in a cooled mode.
Ion implantation methods and apparatus are of rapidly increasing importance to the fabrication of integrated circuits, particularly to the implantation of doping impurities and the like in the fabrication of FET and bipolar integrated circuits. In these circuit technologies, there is also an increasing requirement for high doses of impurities to be achieved in a relatively low cycle time. Although relatively high doses of impurities may be achieved over a relatively short period of time by increasing implant currents, one of the difficulties recently encountered in using high implant currents is the generation of excessive heat in the target disc.
The problem of target disc heat generated by high currents is compounded by the fact that the disc undergoes a heat buildup from cycle to cycle whereby, after a number of processing runs have been made, the target temperature is prohibitively high requiring machine shutdown for cooling purposes.
In addition to necessitating machine shutdown for cooling purposes, the heat buildup in the target also causes the life of the ferrofluidic feedthrough device used to couple the target to the driving motor to be substantially reduced. As a result, not only is machine shutdown necessitated for purposes of cooling the target, but also for purposes of periodically replacing the ferrofluidic feedthrough device. In either event, the down time of the ion implant apparatus due to heat buildup is substantial.
2. Description of the Problem and Prior Art
Although the prior art has addressed the problems incident the use of high implant currents with regard to potential charge buildup due to the accumulation of positive ions on the disc target, the problems due to the accumulated buildup of heat in the target have not, heretofore, been adequately addressed. For a solution to the charge buildup problem on the target disc, reference is made to U.S. Pat. No. 4,118,630 to McKenna et al, entitled "Ion Implantation Apparatus with a Cooled Structure Controlling the Surface Potential of a Target Surface", filed May 5, 1977, and assigned to the assignee of the present invention. It should be noted, that in the McKenna et al solution to the charge buildup problem, McKenna et al employ an electron source adjacent the target which source employs cooling means to maintain the temperature of the source below that of the target disc.
The cooling means comprises cooling conduits, through which pass a coolant such as compressed air or a fluorocarbon coolant. It is clear that such cooling means do not act to, nor are they intended to act to, cool the target disc. Basically, the cooling means of McKenna et al are used to cool a wall positioned between the electrons source and target, which wall is interposed to prevent the electron source from heating the target. Thus, McKenna et al, in attempting to solve the charge buildup problem by utilizing an electron source to neutralize the charges, also find it necessary to take steps to prevent the target from further heating up due to electrons from the electron source impinging upon the target.
For a further description of the manner in which means, such as a wall, or the like, may be positioned between the target disc and the electron source for thereby inhibiting direct rectilinear radiations between the target and source and, therefore, the heating of the target from such source, reference is made to U.S. Pat. No. 4,135,097 to Forneris et al entitled "Ion Implantation Apparatus for Controlling the Surface Potential of a Target Surface", filed May 5, 1977, and assigned to the assignee of the present invention. For a typical example of measuring apparatus reference is made to U.S. Pat. No. 4,011,449 to Ko et al entitled "Apparatus for Measuring the Beam Current of Charged Particle Beam", filed Nov. 5, 1975, and assigned to the assignee of the present invention.